1. Field of the Invention
The present invention relates to a magneto-resistance effect film and a memory using it.
2. Related Background Art
In recent years, semiconductor memories as solid-state memories are frequently used in information technology apparatus and there are various types of memories including Dynamic Random Access Memory (DRAM), Ferroelectric Random Access Memory (FeRAM), flash Electrically Erasable Programmable Read-Only Memory (EEPROM), and so on. The features of these semiconductor memories include both merits and demerits and there exists no memory satisfying all the specifications required by the present information apparatuses. For example, the DRAM has high recording density and the large number of rewritable times, but is a volatile memory, which loses information without supply of power. The flash EEPROM is non-volatile, but it requires a long time for erasing of information and thus is unsuitable for fast processing of information.
In contrast to the semiconductor memories described above, Magneto-Resistance Effect Random Access Memory (MRAM) is a potential memory that can satisfy all the specifications required by many information apparatuses as to recording time, readout time, recording density, the number of rewritable times, power consumption, and so on. Particularly, MRAM making use of the Spin-dependent Tunnel Magneto-Resistance (TMR) effect yields large readout signals and is thus advantageous in achievement of higher recording density or in fast readout, and the practicability thereof as MRAM was substantiated in recent research reports.
The basic configuration of the magneto-resistance effect film used as an element of MRAM is a sandwich structure in which two magnetic layers are adjacently formed through a non-magnetic layer. Materials often used for the non-magnetic film are Cu and Al2O3. The magneto-resistance effect film with the non-magnetic layer made of such a conductor as Cu or the like is called a Giant Magneto-Resistance (GMR) film, and the magneto-resistance effect film with the non-magnetic layer made of such an insulator as Al2O3 or the like is called a Spin-dependent Tunnel Magneto-Resistance (TMR) film. In general, the TMR film demonstrates the greater magneto-resistance effect than the GMR film.
With decrease in element size in order to enhance the recording density of MRAM, the MRAM using in-plane-magnetized films comes to face a problem of failure in retention of information because of influence of demagnetizing fields or curling of magnetization at end faces. In order to circumvent this problem, for example, there is a method of forming the magnetic layers in rectangular shape, but this method does not allow decrease in the element size. Therefore, much improvement in the recording density cannot be expected by that method. A suggestion was thus made to circumvent the above problem by the use of perpendicularly magnetized films, for example, as described in Japanese Patent Application Laid-Open No. 11-213650 (U.S. Pat. No. 6,219,275). In this method the demagnetizing fields do not increase even with decrease in the element size, and it is thus feasible to realize the magneto-resistance effect film in smaller size than the MRAM using the in-plane-magnetized films.
In the case of the magneto-resistance effect film using the perpendicularly magnetized films, the electrical resistance of the magneto-resistance effect film is relatively small in a state in which directions of magnetizations in the two magnetic layers are parallel to each other, but the electrical resistance is relatively large in a state in which the directions of magnetizations are antiparallel to each other, as in the case of the magneto-resistance effect film using the in-plane-magnetized films. FIGS. 1A to 1D are illustrations for explaining the relationship between magnetized states of the magneto-resistance effect film using the perpendicularly magnetized films and magnitude of resistance. In FIGS. 1A to 1D, each magneto-resistance effect film consists of a first magnetic layer (readout layer) 21, a second magnetic layer (recording layer) 23 stacked above the readout layer 21 and made of a perpendicularly magnetized film having a higher coercive force than the readout layer 21, and a non-magnetic layer 22 sandwiched between these layers. Arrows described in the readout layer 21 and the recording layer 23 indicate directions of magnetizations in the respective layers. It is assumed in the present example that the upward magnetization direction in the recording layer 23 represents xe2x80x9c1xe2x80x9d and the downward direction represents xe2x80x9c0xe2x80x9d.
When the directions of magnetizations in the two layers both are upward as shown in FIG. 1A, the electrical resistance of the magneto-resistance effect film is relatively small. When the direction of magnetization in the readout layer 21 is downward and the direction of magnetization in the recording layer 23 is upward as shown in FIG. 1C, the electrical resistance becomes relatively large. Accordingly, when an external magnetic field is applied so as to direct the magnetization upward in the readout layer 21 in the recording state of xe2x80x9c1xe2x80x9d and thereafter another external magnetic field is applied so as to direct the magnetization downward in the readout layer 21, the electrical resistance of the magneto-resistance effect film changes to increase. This change allows the information of xe2x80x9c1xe2x80x9d to be read out. However, the external magnetic fields applied in the readout operation should be of such strength as not to change the direction of magnetization in the recording layer 23. On the other hand, the electrical resistance is relatively large in a state in which the direction of magnetization in the readout layer 21 is upward and the direction of magnetization in the recording layer 23 is downward as shown in FIG. 1B, whereas the electrical resistance is relatively small in a state in which the directions of magnetizations in the two magnetic layers both are downward as shown in FIG. 1D. Accordingly, when the readout operation similar to the above is carried out in the recording state of xe2x80x9c0xe2x80x9d, the electrical resistance changes to decrease. Therefore, this change allows the information of xe2x80x9c0xe2x80x9d to be read out.
Materials mainly used as the perpendicularly magnetized films for the readout layer and recording layer described previously, include alloy films and artificial lattice films of at least one element selected from the rare earth metals such as Gd, Dy, Tb, etc. and at least one element selected from the transition metals such as Co, Fe, Ni, etc.; artificial lattice films of transition metal and noble metal, e.g., Co/Pt and others; alloy films with magnetocrystalline anisotropy in the direction perpendicular to the film surface, e.g., CoCr and others. Among these materials, the amorphous alloys of a rare earth metal and a transition metal are easiest to form the perpendicularly magnetized films and are suitable for use in the memory elements. Particularly, the amorphous alloys containing Gd as the rare earth metal are more preferably applicable to the memory elements, because it is possible to decrease the coercive force and the magnetic field for saturation.
Incidentally, in order to achieve the great magneto-resistance effect, it is necessary to place a magnetic material with a large spin polarization at the interface with the non-magnetic film. There is, however, a problem that the great magneto-resistance effect cannot be attained in the magneto-resistance effect film of three-layer structure in which the non-magnetic film is sandwiched between the magnetic films made of the amorphous alloys containing Gd as described above. A conceivable reason for it is that there exist Gd atoms at the interface with the non-magnetic film. Namely, 4f electrons are responsible for the magnetization of Gd and are different from conduction electrons. When such atoms exist at the interface with the non-magnetic layer, electrons colliding with Gd atoms do not undergo spin scattering in the case of the GMR film or do not undergo spin tunneling in the case of the TMR film. Accordingly, the total magneto-resistance effect appears insignificant in the magneto-resistance effect film.
An object of the present invention is to solve the problems in the conventional art and provide a magneto-resistance effect film with the great magneto-resistance effect and a memory using it.
The above object of the present invention is achieved by a magneto-resistance effect film comprising: a first magnetic layer consisting of a perpendicularly magnetized film; a second magnetic layer consisting of a perpendicularly magnetized film stacked above the first magnetic layer; a non-magnetic layer sandwiched between the first and second magnetic layers; and a first magnetic region formed in granular shape between one of the first and second magnetic layers and the non-magnetic layer and having a spin polarization greater than that of the one of the first and second magnetic layers, wherein the first magnetic region is exchange-coupled with the one of the first and second magnetic layers. The object of the present invention is also achieved by a memory comprising: a memory element consisting of a magneto-resistance effect film; means for recording information in the memory element; and means for reading information recorded in the memory element, wherein the magneto-resistance effect film comprises: a first magnetic layer consisting of a perpendicularly magnetized film; a second magnetic layer consisting of a perpendicularly magnetized film stacked above the first magnetic layer; a non-magnetic layer sandwiched between the first and second magnetic layers; and a first magnetic region formed in granular shape between one of the first and second magnetic layers and the non-magnetic layer and having a spin polarization greater than that of the one of the first and second magnetic layers, wherein the first magnetic region is exchange-coupled with the one of the first and second magnetic layers.